To enhance the reliability of a network, a ring protocol has been proposed. The ring protocol includes a method of keeping nodes configuring a ring in master-slave relationship (for example, refer to RFC3619, Extreme Networks' Ethernet Automatic Protection Switching (EAPS) Version 1, October 2003 (Non-patent Document)). In this method, a master node has a right to control the ring. The master node issues a health check packet via a first port connected to the ring every fixed time so as to check a ring condition. In addition, the master node logically blocks a second port connected to the ring so as to prevent a data packet from entering a loop on the ring. However, the second port does not logically block a control packet including a health check packet. The master node recognizes that a fault occurs on the ring when the master node cannot receive a health check packet via the second port every fixed time.
When a fault occurs on the ring, the master node releases a logical block for the second port. The master node updates a database for data packet transfer in itself immediately after the release. Simultaneously, the master node issues a message for prompting all slave nodes configuring the ring to update a database for transfer. A data packet is transmitted without passing a faulty location on the ring by the update of the database for transfer. The master node also continues to issue a health check packet via the first port while the fault occurs. The master node recognizes that the fault recovers on the ring when the health check packet can be received via the second port. At this time, the master node logically blocks the second port and updates the database for transfer in itself. Simultaneously, the master node issues a message for prompting all slave nodes to update each database for transfer to all the slave nodes configuring the ring. That is, a data packet input to the ring is transmitted on the same line as that before the fault occurs.
In addition, a method that each node configuring a ring and connected via a two-way line issues a health check including packet remaining time in two directions of the ring and updates relation between the node and another node based upon a situation in which a health check issued from another node reaches is disclosed (for example, refer to JP-A No. 234747/2003).
In addition, a method is disclosed that when a node that receives a pilot packet issued from a master node of a ring connected by an optical fiber notifies the master node of whether the node can normally receive the pilot packet or not, the master node identifies a faulty node on the ring (for example, refer to JP-A No. 248316/2004).
In JP-A No. 234747/2003, a relation between a node and a sender node is acquired based upon the remaining time of a packet issued from another node. However, the handling of multiple faults on the ring is not disclosed. In JP-A No. 248316/2004, the master node only checks a condition of another node on the ring and the handling of multiple faults on the ring is not disclosed.
In Non-patent Document, the handling of multiple faults on the ring is not described. Even if multiple faults occur, it is conceivable that a master node in Non-patent Document prompts all nodes configuring the ring to update a database for transfer only when a first fault on the ring occurs and only when the master node recognizes that all faults on the ring recover. Therefore, it is conceivable that when one fault recovers after multiple faults occur on the ring, the master node in Non-patent Document does not prompt all nodes configuring the ring to update each database for transfer. That is, in recovery from multiple faults, it can be estimated that the reliability of data transmission is deteriorated.
Besides, in Non-patent Document, the handling of a fault in the master node on the ring is also not described. Therefore, it is conceivable that a slave node in Non-patent Document cannot prompt the master node to update a database for transfer. Consequently, when a fault occurs in the master node on the ring, the reliability of the network may be deteriorated.